OSPF as a Link-State Protocol• OSPF propagates link-state advertisements rather than routing table updates. – Link = router interface – State = description of an interface and its relationship to neighboring routers• LSAs are flooded to all OSPF routers in the area.• The OSPF link-state database is pieced together from the LSAs generated by the OSPF routers.• OSPF uses the SPF algorithm to calculate the shortest path to a destination.
OSPF Hierarchical Routing • Consists of areas and autonomous systems • Minimizes routing update traffic
Shortest Path First Algorithm• Places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost• Cost = 108/bandwidth (bps)
Configuring Single-Area OSPFRouter(config)# router ospf process-id • Defines OSPF as the IP routing protocolRouter(config-router)# network wildcard-mask area area-id • Assigns networks to a specific OSPF area
Configuring Loopback InterfacesRouter ID • Number by which the router is known to OSPF • Default: The highest IP address on an active interface at the moment of OSPF process startup • Can be overridden by a loopback interface: Highest IP address of any active loopback interface • Can be set manually using the router-id command
Verifying the OSPF Configuration Router# show ip protocols • Verifies that OSPF is configured Router# show ip route • Displays all the routes learned by the routerRouter# show ip routeCodes: I - IGRP derived, R - RIP derived, O - OSPF derived,C - connected, S - static, E - EGP derived, B - BGP derived,E2 - OSPF external type 2 route, N1 - OSPF NSSA external type 1 route,N2 - OSPF NSSA external type 2 routeGateway of last resort is 10.119.254.240 to network 10.140.0.0O E2 10.110.0.0 [160/5] via 10.119.254.6, 0:01:00, Ethernet2E 10.67.10.0 [200/128] via 10.119.254.244, 0:02:22, Ethernet2O E2 10.68.132.0 [160/5] via 10.119.254.6, 0:00:59, Ethernet2O E2 10.130.0.0 [160/5] via 10.119.254.6, 0:00:59, Ethernet2E 10.128.0.0 [200/128] via 10.119.254.244, 0:02:22, Ethernet2. . .
Verifying the OSPF Configuration (Cont.) Router# show ip ospf interface • Displays area ID and adjacency informationRouter# show ip ospf interface ethernet 0Ethernet 0 is up, line protocol is upInternet Address 192.168.254.202, Mask 255.255.255.0, Area 0.0.0.0AS 201, Router ID 192.168.99.1, Network Type BROADCAST, Cost: 10Transmit Delay is 1 sec, State OTHER, Priority 1Designated Router id 192.168.254.10, Interface address 192.168.254.10Backup Designated router id 192.168.254.28, Interface addr 192.168.254.28Timer intervals configured, Hello 10, Dead 60, Wait 40, Retransmit 5Hello due in 0:00:05Neighbor Count is 8, Adjacent neighbor count is 2 Adjacent with neighbor 192.168.254.28 (Backup Designated Router) Adjacent with neighbor 192.168.254.10 (Designated Router)
Verifying the OSPF Configuration (Cont.) Router# show ip ospf neighbor • Displays OSPF neighbor information on a per-interface basisRouter# show ip ospf neighborID Pri State Dead Time Address Interface10.199.199.137 1 FULL/DR 0:00:31 192.168.80.37 Ethernet0172.16.48.1 1 FULL/DROTHER 0:00:33 172.16.48.1 Fddi0172.16.48.200 1 FULL/DROTHER 0:00:33 172.16.48.200 Fddi010.199.199.137 5 FULL/DR 0:00:33 172.16.48.189 Fddi0
Verifying the OSPF Configuration (Cont.)Router# show ip ospf neighbor 10.199.199.137Neighbor 10.199.199.137, interface address 192.168.80.37In the area 0.0.0.0 via interface Ethernet0Neighbor priority is 1, State is FULLOptions 2Dead timer due in 0:00:32Link State retransmission due in 0:00:04Neighbor 10.199.199.137, interface address 172.16.48.189In the area 0.0.0.0 via interface Fddi0Neighbor priority is 5, State is FULLOptions 2Dead timer due in 0:00:32Link State retransmission due in 0:00:03Router# show ip ospf neighbor detailNeighbor 192.168.5.2, interface address 10.225.200.28In the area 0 via interface GigabitEthernet1/0/0Neighbor priority is 1, State is FULL, 6 state changesDR is 10.225.200.28 BDR is 10.225.200.30Options is 0x42 LLS Options is 0x1 (LR), last OOB-Resync 00:03:08 agoDead timer due in 00:00:36Neighbor is up for 00:09:46Index 1/1, retransmission queue length 0, number of retransmission 1First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)Last retransmission scan length is 1, maximum is 1Last retransmission scan time is 0 msec, maximum is 0 msec
OSPF debug CommandsRouter# debug ip ospf eventsOSPF:hello with invalid timers on interface Ethernet0hello interval received 10 configured 10net mask received 255.255.255.0 configured 255.255.255.0dead interval received 40 configured 30Router# debug ip ospf packetOSPF: rcv. v:2 t:1 l:48 rid:188.8.131.52 aid:0.0.0.0 chk:6AB2 aut:0 auk:Router# debug ip ospf packetOSPF: rcv. v:2 t:1 l:48 rid:184.108.40.206 aid:0.0.0.0 chk:0 aut:2 keyid:1 seq:0x0
Summary• OSPF is an interior gateway protocol similar to IGRP, but based on link states rather than distance vectors.• OSPF advertises information about each of its links rather than sending routing table updates like a distance vector protocol does.• Hierarchical routing enables separation of a large internetwork into smaller internetworks, called areas.• The SPF algorithm places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reach that destination.
Summary (Cont.)• The router ospf command starts an OSPF routing process. The network command is used to associate addresses to an OSPF area.• The interface loopback command is used to modify the OSPF router ID to a loopback address.• Any one of a number of show commands can be used to display information about an OSPF configuration.• The debug ip ospf events privileged EXEC command can be used to display information on OSPF-related events, such as adjacencies, flooding information, designated router selection, and SPF calculation.